We assume you are using the latest myServer (not MLServer).
We assume you have the latest apk on all Android devices.
So, there are no known issues with the latest apk talking to the latest myServer.
Since this is affecting multiple devices, it still points to networking problems.
but, you say you can connect via those same device browsers which would eliminate network issues.
Can you put Teamviewer for Android on one of the bad devices and send us the TV user ID / password?
Also, send the TV info for myServer PC.